Standard email just isn't secure. If the internet didn't kill privacy, people and agencies using and abusing it have certainly put it into intensive care. There's nothing we can do to protect the servers our mail goes through, but luckily there is something we can do to protect it before it goes through those servers. We can encrypt it with pretty good privacy (PGP). And open-source system, which means any flaws or back doors are more easily investigated and detected by more people than a closed, proprietary system would be. GPGMail 2 employs PGP, is available for Mac (and Windows), and can help you make sure your emails are only read by the intended recipient.